Refractory concretes, in particular dense refractory concretes, are known for their outstanding properties of resistance to elevated temperatures (ranging from 300° C. to 1800° C.) and for this reason they are especially used for coating the furnaces in the iron and steel industry or for other applications under elevated temperatures. Indeed, furnaces must be able to withstand aggressive conditions of thermal, mechanical or chemical nature.
In general, refractory concrete is prepared by mixing, in a blending device, a refractory aggregate (tabular alumina, corundum, bauxite, magnesia, alumina silicates, dolomite, etc.), an aluminous binder, optionally ultrafine particles, such as silica fume or alumina powder, even one or more additives, such as a molding additive and mixing water. Once mixed, a fresh concrete is obtained which is easy handling and will be used so as to form the expected structure. The latter will then be left to harden upon drying. During this drying, the refractory concrete develops a certain amount of mechanical resistance (cure period). Then this hardening phase gives rise to a dehydration phase in the refractory concrete. This one leads to the removal of free water, as well as of crystallization water.
However, it appeared that the drying step due to an increase in temperature became problematic to prepare refractory concretes and in particular dense refractory concretes. This specific density certainly improves the corrosion resistance of refractory concretes, but it causes a problem in the drying step, since such specific density goes along with a poor permeability, which interferes with water drainage, that is to say with the removal of free water and crystallization water.
Indeed, the water amount that may be drained away by a material forming a sealed enclosure depends on the permeability of such material. It should be distinguished with the water amount, which try to escape from a material forming a sealed enclosure, at a given temperature and at a given pressure.
In general, for economic reasons, the drying step of refractory concretes has to be as quick as possible. To this end, the concretes often have to be heated, for example to a temperature coming close to 300° C. However, when the water amount which tries to escape from the refractory concrete is higher than the water amount actually drained away, then there is an explosion hazard. Indeed, free water and crystallization water that are removed lead to the formation of steam. If the temperature increase is too fast during the drying step, the steam pressure may exceed the mechanical resistance of the thus formed concrete and cause the explosion of the latter.
To date, to limit explosion hazards during the drying step of refractory concretes, a first solution consists in providing said refractory concretes with softer and slower heat cycles. These heat cycles extend the drying time and are economically not advantageous.
A second solution consists in modifying the permeability of the material, without excessively affecting its porosity, so as not to potentially embrittle said material.
Porosity corresponds to the volume of void existing within a material and the permeability corresponds to the manner with which these voids are arranged to each other. Increasing the porosity often results in a decrease in the mechanical resistance of a material, whereas increasing its permeability enables water to be drained away more easily upon drying.
One of the existing solutions to improve the permeability of the material consists in using polymer fibers (such as polypropylene or polyvinyl, etc.). However, this solution is only effective if the heating temperatures do exceed the fiber melting temperature. This solution does not reduce the explosion hazard, which appears as soon as the temperature of the material exceeds 100° C. Moreover, said fibers are difficult to disperse homogeneously within a dry concrete mix, that is to say wherein water has not yet been added thereto. The material which then becomes heterogeneous, has areas with a high explosion hazard. Lastly, to preserve the castability of a concrete comprising such fibers, it is necessary to add more water to the concrete formulation. But increasing the water amount in the formulation of a refractory concrete results in an increased porosity in the refractory concrete after drying. As a consequence, such a refractory concrete has a poor final quality.
A third solution consists in adding aluminum metal to the refractory concrete initial formulation. Indeed, aluminum metal hydrolyzes concomitantly with a pH value increase triggered by the hydration of said concrete. This hydrolysis reaction releases hydrogen which, bubbling through the material, creates outlet channels. Such outlet channels are used to drain water away during the drying of the refractory concrete. However this solution presents a non-negligible risk of explosion of the hydrogen released during the implementation of voluminous parts in a confined space.
Therefore, there is a real need for new adjuvants developed for cement compositions and/or for refractory concrete compositions enabling to limit the explosion risks during refractory concrete fast drying, while better preserving the final properties of said refractory concrete formed, especially its compressive strength, its reliability, etc., and this, without affecting its rheology. Indeed, it is desirable that the rheology of the thus formed refractory concrete, especially the handling (consistency) and the workability thereof, be preserved against the addition of a new adjuvant.
The aim of the present invention is thus to provide a new adjuvant for a cement or a refractory concrete composition, in particular a dense refractory concrete, avoiding at least partially the abovementioned drawbacks.